In last night's two-hour season finale, Emma and Hook find themselves trapped in the past, leading to consequences that could change their future. When they attempt to enter a portal that can bring them back to Storybrooke, Emma accidentally takes a dark magical object along with her.
At the end of the episode, the object releases a blue liquid that transforms into none other than Frozen's QUEEN ELSA! Except in this world, she appears to be the next villain, or perhaps the antidote to Regina's recurring evil side.
It is not clear if the role has been cast or if this was simply a stand-in, but one thing is certain: Disney (who owns ABC) is taking advantage of all the Frozen success.